After entering the Yablochinsky Monastery in the Diocese of Kholm, Arkady was tonsured a monk on June 7, 1907 by Bishop Eulogius of Kholm]] and given the name Sergius. Ordained a hieromonk in 1908, he joined into the missionary program of the monastery, mainly among the Uniates of the area. In 1914, Father Sergius was elevated to the dignity of archimandrite and appointed assistant abbot of the monastery.

With the start of World War I in 1914, the monastery was evacuated as it was near the battle line between the Russian and Austro-Hungarian Empires. After the war ended, Archim. Sergius was consecrated in 1920 to the episcopate as Bishop of Belsk, a see that, following the peace treaties, was part of the nation of Poland. As Bp. Sergius opposed the proposed autocephaly of the Orthodox Church in Poland, he was arrested in 1922 and deported from Poland to Czechoslovakia where Metropolitan Eulogius of Western Europe under ROCOR assigned Bp. Sergius as his vicarBishop of Prague.

Bp. Sergius remained with Metr. Eulogius during the following decades as, in 1927, Eulogius split with ROCOR and recognized the Moscow Patriarchate. Bp. Sergius also remained with him when Metr. Eulogius was accepted under the canonical care of the Ecumenical Patriarch in 1930 after the Moscow Patriarchate had deposed Metr. Eulogius. In 1945, as World War II ended, Bp. Sergius joined again with Metr. Eulogius as he re-entered communion with the Moscow Patriarchate.

Following the death of Metr. Eulogius in 1946, Bp. Sergius remained loyal to Moscow. On April 17, 1946, he was elevated to Archbishop of Prague. Then, on June 7, 1946, Abp. Sergius was appointed Archbishop of Vienna of the Western European Diocese of the Moscow Patriarchate and, in October 1946, he was named Exarch of the Patriarchate's Central European District. On November 16, 1948, Abp. Sergius was appointed Archbishop of Berlin and Germany.

On September 20, 1950, Abp. Sergius was appointed Archbishop of Kazan and Chistopol. In Kazan, he related closely with the people. To meet more people he took different routes when he walked to the Cathedral and engaged the poor people in conversation. He joined them in their homes and, after learning of their situations, he would leave sums of money in their house.